Compound semiconductors and a method for thin film growth

ABSTRACT

A molecular beam epitaxy (MBE) system (10) is provided to grow thin film, epitaxy layers (44, 46, 48, 50) on compound semiconductor substrates (40). A mass spectrometer detector (95) is used to monitor and control the flux from selected sources (21, 23, 25, 27) within the MBE system (10). A uniform layer of indium gallium arsenide (46, 50) may be grown on a semiconductor substrate (40) by controlling the indium flux with respect to substrate (40) temperature and time. An epitaxy layer (46) of indium gallium arsenide with uniform mole fraction concentration and reduced lattice strain is produced.

This is a division of application Ser. No. 07/958,888, filed Oct. 9,1992, now U.S. Pat. No. 5,400,739.

TECHNICAL FIELD OF THE INVENTION

This invention relates to semiconductor compounds and more particularlyrelates to equipment and methods for producing thin film layers on acompound semiconductor substrate.

BACKGROUND OF THE INVENTION

Integrated circuits have become more complex and smaller in scale as theapplications for using integrated circuits have rapidly multiplied.Design requirements for such integrated circuits frequently exceed thecapabilities of traditional silicon based semiconductor compounds. Thesearch for higher performance, higher capacity and better frequencycharacteristics has resulted in developing new semiconductor compoundsformed from elements in Group III and Group V of the Periodic Table.Combining elements from Group III and Group V has produced a new classof compound semiconductors which is particularly useful in high speedmicrowave and optical devices. Gallium (Group III) and arsenic (Group V)have been combined to produce gallium arsenide (GaAs) compoundsemiconductors.

A substantial amount of testing and development has been conducted withrespect to gallium arsenide compound semiconductors. Various elementssuch as aluminum (Al) and indium (In) can be added to gallium arsenideto produce abrupt changes in band gap energies and refractive index.Aluminum gallium arsenide (AlGaAs) and gallium arsenide have nearlyidentical lattice constants which allows relatively thick layers(several microns) of aluminum gallium arsenide to grow on a galliumarsenide substrate. This characteristic results in gallium arsenide andaluminum gallium arsenide compounds being widely used in fabricatingsemiconductor devices. Indium gallium arsenide (InGaAs) has asignificantly larger lattice constant as compared to gallium arsenide.

Another problem associated with present systems and methods for growingindium gallium arsenide epitaxy layers is that at high temperatures(500° C. and greater), the indium gallium arsenide layer generally has ahigher mole fraction of indium at the interface with gallium arsenideand a lower mole fraction of indium at the interface with aluminumgallium arsenide. This change in indium concentration, related toevaporation and desorption of indium at higher temperatures whilegrowing epitaxy layers, produces undesirable electrical characteristics.Since indium gallium arsenide has a bigger lattice constant as comparedto gallium arsenide, stresses are present at the interface bond betweenthe indium gallium arsenide layer and the gallium arsenide substrate.Changes in the mole fraction of indium and discontinuities in thelattice structure at the bond interface have previously limited orminimized the use of indium gallium arsenide in preparing epitaxy layersfor semiconductor devices.

The first step in fabrication of an integrated circuit on asemiconductor chip is to grow a relatively large, single crystal oringot from the desired semiconductor compound. Various techniques arecommercially available for growing a single crystal from galliumarsenide compounds. These techniques are sometimes referred to as bulkgrowth procedures.

A large, single crystal or ingot of gallium arsenide will typically havedimensions of three to five inches in diameter and two to three feet inlength. This large crystal is then sliced into thin wafers which providea crystalline substrate of compound semiconductor material. One or morethin film layers (sometimes referred to as "epitaxy layers") are thendeposited on the crystalline substrate to produce the electricalcharacteristics associated with semiconductor devices and integratedcircuits. Various compounds and elements may be included within the thinfilm layers to modify the band gap energy and the refractive index ofthe layers as compared with the semiconductor material in the substrateand with other layers on the substrate.

Various techniques have previously been used to place thin film layerson semiconductor substrates. Examples of thin film technologies whichhave previously been used include liquid phase epitaxy, chemical vapordeposition, sputtering and vacuum evaporation. Molecular Beam Epitaxy(MBE) has been found to be a particular useful technique for placing athin film layer on compound semiconductors such as gallium arsenide.With an understanding of surface physics and by observing variations insurface structure resulting from the relationship between arrival of anatom (beam flux) and substrate temperature, MBE allows preparation ofhigh quality, thin film layers by adding one atomic layer upon the nextatomic layer. This type of thin film layer growth is particularlyadvantageous in fabricating gallium arsenide semiconductors.

General background information on molecular beam epitaxy with respect todevelopment of semiconductor materials and particularly with respect togallium arsenide may be found in "The Technology and Physics ofMolecular Beam Epitaxy" by E. H. C. Parker, published by Plenum Press in1985. This book teaches the use of a mass spectrometry detector tomonitor background levels for selected atoms or molecules in an MBESystem. Mass spectrometers used for this purpose are sometimes referredto as residual gas analyzer probes. Mass spectrometers have also beenused in MBE Systems to detect undesired leaks from sources in the MBESystem.

A substantial amount of work has been conducted to develop galliumarsenide semiconductors with epitaxy layers formed from aluminum galliumarsenide (AlGaAs) compounds. The use of indium gallium arsenide (InGaAs)as a ternary compound to grow an epitaxy layer on gallium arsenidesubstrates along with an aluminum gallium arsenide epitaxy layer offerssubstantial electrical advantages in integrated circuit design forfabrication on a semiconductor chip as compared to only an aluminumgallium arsenide epitaxy layer.

Therefore, a need has arisen for equipment and methods to grow highquality indium gallium arsenide epitaxy layers on a gallium arsenidesubstrate with uniform strain characteristics in the lattice structureand uniform mole fraction composition in the surface and subsurfacelayers of the semiconductor chip.

SUMMARY OF THE INVENTION

In accordance with the present invention, disadvantages and problemsassociated with growing thin film layers of indium gallium arsenide on agallium arsenide substrate have been substantially reduced oreliminated.

Mass spectrometry monitoring is used with a molecular beam epitaxysystem to grow thin film layers on a compound semiconductor substratesuch as gallium arsenide. A mass spectrometry detector is positionedwithin the MBE system to detect both background signals for a selectedatom and signals reflected from the substrate surface. The resultingepitaxy layers have a strained heterostructure lattice and uniform molefraction composition.

In accordance with one aspect of the present invention, a massspectrometry detector is placed within the molecular beam epitaxy systemto monitor molecular growth on a compound semiconductor substrate.Indium gallium arsenide layers which are uniform in both composition andlattice structure are grown on the substrate by varying the indium fluxand substrate temperature with respect to time.

The present invention has significant technical advantages in that itprovides a method for growing high quality indium gallium arsenide andaluminum gallium arsenide layers on gallium arsenide to produce lownoise semiconductor devices for high power applications. Using in situmass spectrometry, the indium flux and substrate temperature are variedwithin a molecular beam epitaxy system to produce the high qualityindium gallium arsenide layer.

Another significant technical advantage of the present invention is theuse of a mass spectrometry detector within a molecular beam epitaxysystem to grow an epitaxy layer with a uniform mole concentration ofindium gallium arsenide throughout the layer and a uniform strainedheterostructure at the interface between indium gallium arsenide andgallium arsenide. Varying the indium flux in response to informationprovided by the mass spectrometry detector will compensate forvariations in the indium incorporation rate within the epitaxy layer.The advantages of the present invention may be extended to use ingrowing other strained layers within a molecular beam epitaxy system.

A further technical advantage of the present invention is producing highquality aluminum gallium arsenide/indium gallium arsenide/galliumarsenide strained hetero-structures with uniform indium mole fractionusing in situ mass spectrometry monitoring. The present invention allowsgrowing epitaxy layers with optimum indium mole fraction concentrationand optimum thickness of the indium gallium arsenide layer duringfabrication of a semiconductor device. By varying the indium flux ratewith respect to substrate temperature and time, uniform incorporation ofindium is produced in the desired epitaxy layer. The loss of indium fromthe epitaxy layer during temperature cycling for growth of aluminumgallium arsenide has been substantially eliminated by adding a thin caplayer of gallium arsenide or aluminum gallium arsenide.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, references is now made to the following descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic plan view with portions broken away of a molecularbeam epitaxy (MBE) system incorporating the present invention;

FIG. 2 is a schematic drawing in section with portions broken away of acompound semiconductor substrate and epitaxy layers fabricated inaccordance with the present invention;

FIG. 3 is a graph of the band gap energy near the hetero-structureinterface between AlGaAs, InGaAs, and GaAs;

FIGS. 4a and 4b are schematic drawings showing typical latticestructures for a compound semiconductor substrate and epitaxy layer;

FIG. 5 is a graph showing an indium signal versus time;

FIG. 6 is a graph showing an indium signal versus time; and

FIG. 7 is a graph of an indium signal which has been produced inaccordance with the present invention in the MBE system of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiment of the present invention and its advantages arebest understood by referring to FIGS. 1 through 7 of the drawings, likenumerals being used for like and corresponding parts of the variousdrawings.

The principle components of Molecular Beam Epitaxy (MBE) System 10,shown in FIG. 1, are contained within chamber 12. Wall 14 of chamber 12is designed to maintain an ultra high vacuum (less than 10⁻¹⁰ Torr)within chamber 12. The additional components of MBE System 10 aresurrounded by cryogenic paneling 16 which is preferably filled withliquidnitrogen. FIG. 1 shows only a single ultra high vacuum chamber 12.In actual practice, a molecular beam epitaxy system may contain aplurality of ultra high vacuum chambers (not shown) which areinterconnected to communicate with each other through appropriatelydesigned gate valves (not shown).

Substrate holder 18 is contained within chamber 12 to hold substrate 40on which a plurality of epitaxy layers may be grown. Substrate holder 18willpreferably include equipment to rotate substrate 40 and to heatsubstrate 40 to selected temperatures depending upon the characteristicsof the semiconductor compounds used to produce substrate 40 and theepitaxy layers. A plurality of cells 20, 22, 24, 26 and 28 are providedwithin chamber 12. Shutters 30, 32, 34, 36 and 38 are used to controlcommunication with their respective cells 20, 22, 24, 26 and 28. Ifdesired, each cell 20, 22, 24, 26 and 28 could be a separate smallvacuum chamber attached to chamber 12. Ionizing beam sources 21, 23, 25and 27 are disposed within cells 20, 22, 24 and 26 respectively. Sources21, 23, 25 and 27 produce an atomic or molecular flux 21f, 23f, 25f, and27f respectively depending upon the characteristics of each source. Theflux beams are used to grow the desired epitaxy layers on substrate 40in MBE System 10.

Cell 28 contains mass spectrometer detector 95. Various types of massspectrometers satisfactory for use with the present invention arecommercially available. Mass spectrometer detector 95 is preferably acompact quadrapole detector sized to fit within cell 28. Shutter 38 maybeopened and closed to allow mass spectrometer detector 95 to monitorselected atoms or molecules from gases within chamber 12. An importantelement of the present invention is mounting mass spectrometer detectorinsitu in chamber 12 to receive signals directly from the surface ofsubstrate 40 in addition to background signals within chamber 12.

When shutter 38 is opened, detector 95 will measure both backgroundlevel for a selected gas molecule or atom in chamber 12 and signals forthe selected molecule or atom received from the surface of substrate 40.The received signal includes reflection of flux beams 21f, 23f, 25f or27f from the surface of substrate 40 if the respective flux beamincludes the selected atom or molecule. The received signal fromsubstrate 40 will alsoinclude atoms which have been desorbed orevaporated from epitaxy layers onsubstrate 40. Elements such as indiummay segregate into "pools" at the surface of an epitaxy layer resultingin a high evaporation rate at elevated temperatures. The graphs shown inFIGS. 5, 6, and 7 are based on signals from mass spectrometer detector95 sent to an RF Analyzer (not shown).

FIG. 2 shows a schematic representation of substrate 40 which may besecured to substrate holder 18 of MBE System 10. Various semiconductorcompounds could be used to produce substrate 40. The present inventionwill be described with respect to gallium arsenide compoundsemiconductors. Substrate 40 is formed by slicing a thin wafer from alarge, single crystal (not shown) of gallium arsenide. One or morerelatively thick (approximately one micron) buffer layers 42 of galliumarsenide are then formed on top of substrate 40 to correct for anyirregularities or deformities in the surface of substrate 40 resultingfrom slicing the thin wafer and mounting substrate 40 on substrateholder 18. Fabricating buffer layer 42 may require over sixty percent ofthe total time that substrate 40 is within MBE System 10.

Layer 44 of indium gallium arsenide may then be deposited on bufferlayer 42 in accordance with the present invention. An important featureof the present invention includes adding a cap layer of gallium arsenide46 on top of indium gallium arsenide layer 44. Cap layer 46 isrelatively thin, approximately ten angstroms, as compared to the otherepitaxy layers on substrate 40. Aluminum gallium arsenide may also beused to grow cap layer46. Layer 46 cooperates with other features of thepresent invention to maintain a uniform concentration of indium withinepitaxy layer 44 by limiting desorption of indium when the temperatureof substrate 40 is raised to grow aluminum gallium arsenide layer 48.

Layer 48 may be formed from aluminum gallium arsenide and include otherdoping materials as required for the specific integrated circuit whichwill be fabricated on the semiconductor device. An optional cap layer ofgallium arsenide 50 may then be placed upon epitaxy layer 48 as requiredby the circuit design or to prevent undesired oxidation of aluminum fromlayer 48. MBE System 10 may be used to grow a plurality of layers 42,44, 46, and 48 on substrate 40 as a continuous process without requiringremoval of substrate 40 from holder 18. The specific number of indiumgallium arsenide layers and aluminum gallium arsenide layers grown inMBE System 10 will depend upon the specific integrated circuit design.

During the process of forming epitaxy layers of indium gallium arsenide44 and aluminum gallium arsenide 48 on substrate 40, substrate holder 18and substrate 40 will typically be heated to selected temperatures inthe range of 400° to 700° C. At elevated temperatures (greater than 500°C.), arsenic has a tendency to decompose from the surfaceof substrate 40and from layers 42 44, 46, 48, and 50. The partial pressureof arsenicwithin chamber 12 will typically be five to ten times higher than thepartial pressure of gallium and other Group III elements within chamber12 to suppress the tendency of arsenic to decompose from surface layerson substrate 40. With an appropriate overpressure of arsenic in chamber12, cell 20 containing arsenic source 21 may be opened by shutter 30 andcell 22 containing gallium source 23 may be opened by shutter 32 togrowbuffer layer 42 on substrate 40 using MBE System 10.

Cell 24 preferably contains indium source 25 and cell 26 containsaluminum source 27. In order to grow a layer of indium gallium arsenideon substrate 40, shutter 34 is open to exposed indium source 25 alongwith opening shutter 30 for arsenic source 21 and shutter 32 for galliumsource23. In the same manner, when growing an aluminum gallium arsenideepitaxy layer, shutter 36 will be opened for aluminum source 27. Duringthe fabrication of an integrated circuit, various doping compounds mayalso beplaced within selected epitaxy layers. Therefore, MBE System 10is not limited to only five cells as shown in FIG. 1. Additional cellsmay be added for each element or compound which will be added to anepitaxy layeron substrate 40.

The temperature of substrate holder 18 and substrate 40 is varied inaccordance with the present invention depending upon the type of epitaxylayer which is being grown on substrate 40. The temperature for eachcell may vary between 1000° C. to 1600° C. depending upon the type ofelement or compound source within the respective cells. Increasingthetemperature of a cell will generally increase the atomic or molecularbeam flux emitted by each source from its respective cell. In the samemanner, reducing the temperature of a cell will generally reduce thebeam flux associated with the specific source and cell.

Relatively thin layers of indium gallium arsenide may be grown attemperatures of 500° C. and less. The mole concentration of indium in anepitaxy layer grown at this lower temperature is generally uniform butthe lattice structure will have defects and discontinuities. Therefore,the resulting thin layer has electrical properties which are less thandesired. Indium gallium arsenide layers grown in the temperaturerange of500°-550° C. have optimum electrical characteristics. However, at thesehigher temperatures, the indium concentration becomes less uniform andincreased strain is noted within the lattice structure. The optimumtemperature for growing an aluminum gallium arsenide epitaxy layer is620° C. or higher. At these elevated temperatures a significant changein mole concentration of indiumwill occur. Therefore, without thepresent invention the optimum temperature (500°-550° C.) for growing anindium gallium arsenide epitaxy layer is not compatible with the optimumtemperature (≧620° C.) for growing an aluminum gallium arsenide epitaxylayer.

FIG. 3 is a representation of the relative bandgap energies of galliumarsenide, indium gallium arsenide, and aluminum gallium arsenide. Sincethe bandgap of indium gallium arsenide is substantially less than eitheraluminum gallium arsenide or gallium arsenide, indium gallium arsenidecanbe used to significantly improve carrier confinement and transportproperties of the resulting semiconductor device. The benefits of usingindium gallium arsenide have been well known. Increased indium moleconcentration reduces bandgap energy and produces better semiconductorcharacteristics. Increased indium concentration unfortunately alsoproduces increased strain in the lattice structure. Differences inlatticeconstant and optimum temperature for epitaxy layer growth haveprevented effective fabrication of indium gallium arsenide layers withlayers of gallium arsenide and aluminum gallium arsenide.

FIG. 4a shows interfacial dislocations at 82, 84, and 86 and theresulting strain which could occur between gallium arsenide layer 42 andindium gallium arsenide layer 44. The dislocation or misfit between thelattice structure at the epitaxy layer interface will cause increasedstrain and will potentially limit the thickness of indium galliumarsenide layer 44. Defects such as 82, 84, and 86 at the strained layerinterface may trap carriers and produce undesirable electricalcharacteristics in the finished semiconductor device. The strainedlayers represented by FIG. 4a will prevent growing an indium galliumarsenide layer of optimum thickness. Defects at the interface betweenthe epitaxy layers and the substrate degrade the quality of the epitaxyfilm and result in reduced semiconductor capabilities.

FIG. 4b shows a more uniform elastic strain between indium galliumarsenidelayer 42 and indium gallium arsenide layer 44 resulting fromgrowing epitaxy layer 44 in accordance with the present invention.Dislocations and misfits which can result from variations in indiumconcentration in epitaxy layer 44 are reduced or eliminated. The presentinvention minimizes discontinuities and deformities in the latticestructure at the interface of epitaxy layers 42, 44, 46, 48 and 50.

By using mass spectrometry detector 95 in MBE System 10, it is possibleto measure and record a signal for a selected atom or molecule receivedfrom substrate 40 and background level in chamber 12 versus time. FIG. 5shows the indium signal versus time within chamber 12 as measured bydetector 95. FIG. 5 demonstrates that when substrate holder 18 andsubstrate 40 areheated to 500° C. or less, indium can be deposited onsubstrate 40 at a relatively uniform rate with respect to time. Theindium signal measured from time zero to t₁ represents the backgroundlevel of indium within chamber 12. At time t₁, shutter 34 is opened toprojectindium beam flux 25f from source 25 into chamber 12 directed atsubstrate 40. With the temperature of substrate 40 at 500° C. the indiumsignal within chamber 40 is relatively flat between times t₁ and t₂.This flat curve indicates low reflection and desorbtion and a uniformlow rate of indium growth in epitaxy layer 44 on substrate 40. At timet₂ shutter 34 is closed and the indium signal in chamber 12 rapidlydecays to a new background level. The background level at t₂ is slightlyhigher than at t₁ as a result of incorporating a small amount of indiuminto epitaxy layer 44 on substrate 40.

As previously noted, at temperatures less than 500° C., the indiumgallium arsenide layer grows at a relatively low rate, is thin and hasvery poor electrical properties. High quality indium gallium arsenideepitaxy layers are generally produced when substrate holder 18 andsubstrate holder 40 are heated to temperatures between 500° C. and 550°C. A more uniform strained heterostructure is produced in thiselevatedtemperature range.

FIG. 6 demonstrates the increase in indium signal which occurs atsubstratetemperatures between 500° C. and 550° C. The increasedindiumsignal detected by mass spectrometer 95 shown in FIG. 6 shows ahigher evaporation or desorbtion rate of indium and thereforepotentially lower incorporation of indium into epitaxy layer 44. Duringthe first time interval zero to t₁, shutter 34 for indium source 25 isclosed and detector 95 records only the background indium-signal inchamber 12. At time t₁, shutter 34 for source 25 is opened to emitindium flux 25f into chamber 12. From time t₁ to t₂ the indium signalincreases at a relatively rapid rate indicating high reflection anddesorbtion of indium from epitaxy layer 44. From time t₂ to time t₃, theindium signal increases only slightly at a more uniform rate indicatinga steady state condition for indium incorporation within epitaxy layer44. At time t₃, shutter 34 for source 25 is closed and the indium signaldecreases to normal background. The decay in the indium signal after t₃is substantially different than the decay in the indium signal shown inFIG. 5. The gradual decrease in the indium signal and the overallhigherbackground signal indicate that a substantially greater amount of indiumhas been deposited in epitaxy layer 44.

The result of varying the temperature of substrate 40 and the indiumflux from source 25 to produce a stair step pattern is demonstrated byFIG. 7. Indium gallium arsenide epitaxy layer 44 is preferably grownusing the stair step pattern shown in FIG. 7. The initial portion ofepitaxy layer 44 is grown with substrate 40 at a temperature of 500° C.or less. The indium flux from source 25 will be at a relatively lowlevel with a low rate of indium evaporation from substrate 40 duringthis first time interval. After the first time interval, the temperatureof substrate 40 is increased to 530° C. at time t₁ which causes asignificant increase in the indium evaporation and desorbtion rate atthe surface of layer 44. Therefore, a higher indium flux from source 25is required for layer 44 to continue steady growth with uniform indiumconcentration. The flux from indium source 25 will preferably beincreased at this time. By changing the temperature of cell 24 andindium source 25 at time t₁ to correspond with changes in thetemperature of substrate 40, the concentration of indium will be moreuniform and maintained at the desiredlevel as layer 44 continues togrow.

At t₂ the temperature of substrate 40 may be increased to a higher valuesuch as 550° C. and the temperature or flux from indium source 25 alsoincreased. Again, the higher indium signal indicates a higher level ofindium is being received from epitaxy layer 44. By using the stair steppattern with both the temperature of substrate 40 and flux from source25, a uniform indium concentration with the desiredelectricalcharacteristics is produced in epitaxy layer 44 with uniformedlattice strain as compared to previous procedure for growing epitaxylayers.

Mass spectrometry detector 95 allows optimizing the changes in thetemperature of substrate 40 and the flux from source 25 to produce thedesired mole fraction of indium within epitaxy layer 44 and desiredthickness of layer 44. A typical time period for exposure of the indiumsource would be approximately 60 seconds. By maintaining a higher fluxat selected time periods, indium will incorporate into epitaxy layer 44and produce a more uniform indium concentration. The flux from the othercellsis maintained relatively constant during the growth of epitaxylayer 44. Only flux 25f from indium source 25 is varied. Dotted lines 62and 64 in FIG. 7 represent changes in the indium signal which may becaused by changing the temperature of substrate 40 and indium flux 25ffrom source 25 to achieve the desired electrical characteristics in thefinal semiconductor device. Mass spectrometer detector 95 allowsadjusting the flux level and time intervals for optimum growth of theepitaxy layers with the desired electrical characteristics.

The present invention may be effectively used to grow epitaxy layerscomprising other ternary compounds selected from elements in Group IIIandGroup V of the periodic table and is not limited to growing indiumgallium arsenide epitaxy layers. The present invention is particularlyuseful withternerary compounds subject to strained layer growth andvariations in the incorporation rate of Group III elements in theepitaxy layer.

Although the present invention has been described in detail, it shouldbe understood that various changes, substitutions and alterations can bemadewithout departing from the spirit and scope of the invention asdefined in the following claims.

What is claimed is:
 1. A system for growing epitaxy layers on a compoundsemiconductor substrate, said system comprising:a vacuum chamber havinga substrate holder; a plurality of cells to hold atomic and/or molecularsources; means for selectively opening and closing said cells; a massspectrometry detector positioned to sense the level of atoms ormolecules in said chamber, said detector generating a signalproportionate to said level of atoms or molecules; and memos foradjusting the flux from said cells and the temperature of said substrateholder in response to said signal.
 2. The system for growing epitaxylayers as defined in claim 1, wherein said mass spectrometry detector isinside said chamber.
 3. The system for growing epitaxy layers as definedin claim 1 further comprising the mass spectrometry detector positionedto receive signals reflected from the surface of the semiconductorsubstrate.
 4. The system for growing epitaxy layers are defined in claim1 wherein one of said plurality of cells contains indium.
 5. A systemfor forming thin film layers on a semiconductor substrate, said systemcomprising:a temperature-controllable substrate holder; a plurality ofatomic and/or molecular sources; a mass spectrometer positioned to senseatoms or molecules emitted from said sources; means for varying thetemperature of said substrate holder and the flux of at least one ofsaid sources in response to a signal from said mass spectrometer.
 6. Thesystem of claim 5 wherein at least one of said sources is a cellcontaining indium.
 7. The system of claim 5 wherein said substrateholder, said sources, and said mass spectrometer are inside a vacuumchamber.